Method statement for cement bentonite slurry wall

Fixing of alignment for Diaphragm Wall:
Alignment of Diaphragm Wall shall be fixed on the ground with the help of total station as per the relevant drawing. Proper alignment of Diaphragm Wall shall be maintained by means of guide wall.

Construction of guide wall:
Guide wall of 1.20 m depth shall be made of reinforced cement concrete as shown in the relevant drawing. Distance between internal faces of guide wall shall be maintained 630mm. Dimensions of guide wall, whether in filled area or in excavation shall be same as specified in the above-referred drawing. Filled portion shall be compacted properly and then only excavation activity for guide wall shall be started.

Construction of Diaphragm Wall:
Construction of Diaphragm Wall shall be done by alternate panel method. In this method primary panels shall be cast first leaving suitable gaps in between. Secondary panels & closing panel shall then be cast in this gap. The length of panel may vary from one location to another location depending upon the cut-off level and the termination depth. The changes shall be intimated in advance. Two stop end tubes shall be used at the ends of primary/secondary panels to support Cement Bentonite Slurry and to form suitable joints with the adjacent panel. The shape of the secondary panel end shall be such as to form proper joint with primary panel. 580 mm dia. MS hollow pipes shall be used as stop end tube. The semicircular shape shall provide the proper joint between adjacent panels. Length of panels shall be marked on the guide wall prior to start of trenching work.

Boring / Trenching: Boring shall be carried out by wire rope grab operated by means of suitable crane. The crane shall control the vertical motion of grab. Initially grabbing/ boring shall be done from guide wall top up to water table level in dry conditions. After completing the dry boring, the bentonite slurry of minimum density 1.04 gm/cc shall be pumped to the partly excavated trench and bore shall be filled with bentonite slurry, and further grabbing shall be continued up to specified depth as mentioned in the relevant drawing. Continuous supply of bentonite slurry shall be maintained in bore during entire trenching process to have a constant head above the ground water level. Sodium based bentonite available at Bhuj/Bhavnagar shall be used for bentonite slurry. Necessary testing like density, PH value, viscosity shall be carried out at site until a consistent working pattern is established, taking into account the mixing process, blending of freshly mixed bentonite with previously used slurry etc. Contaminated bentonite shall be removed from site / bentonite tank by means of tankers / tractors and shall be disposed off at suitable location shown by client / consultant. Usually panel width shall vary from 4.0 m to 6.0 m, so that the entire trenching shall be completed in three cuts as per grab size. Panel width may vary as per site conditions. During trenching operation adequate care shall be taken to keep the bore vertical to such an extent so that it meets the permissible limits specified in the technical specification. To maintain verticality of trench following measures shall be adopted: Crane shall be placed on relatively firm and leveled ground and level of track chain of crane shall be checked by means of sprit level. During trenching grab shall be rotated by 180 degree after every three to four grabbing operation. To check the verticality distance between grab rope and guide wall face shall be monitored. After completion of boring up to specified depth, the boring equipment shall be shifted to another location.

Fixing of stop end tubes: Stop end pipes shall be lowered up to the bottom of trench maintaining pre specified distance marked on the guide wall. The center of stop end tubes shall match with the panel markings on Guide wall. Shutter release oil shall be applied on stop end tubes. During placing of stop end pipes, verticality shall be monitored by means of plumb bob to make the same vertical enough to meet the requirements specified in the technical specification.

Cleaning of excavated Trench: After trenching is completed, for flushing tremmie pipe of 200 mm dia shall be lowered in the trench keeping a gap of 300 mm to 500 mm from bottom of trench. Bentonite shall be pumped through the tremmie pipe so as to clean the bottom of trench. Bentonite level shall be maintained in trench by continuously pouring Bentonite from the Bentonite tank. Care shall be taken to avoid Bentonite level from going below bottom of Guide wall. Flushing shall be continued unless Bentonite density comes 1.15 gm/cc or below 1.15 gm/cc. For checking purpose the suitable bentonite sampler shall be used for collecting sample from the bottom of trench.

Forming of Cement-Bentonite Slurry Diaphragm wall: After Flushing of Excavated trench re-circulation to be started for replacement by the Cement bentonite slurry with the help of suitable capacity of Vacseal/Electric Operated Pump s and Venture Mixer using high speed agitators. The Pumping out of Bentonite Slurry is to be done from the bottom of the trench and the Cement –Bentonite Slurry is to flow into trench by Gravity. Pumping out of the Slurry from the excavated Trench to Mixing tank and flow back to Trench till continue and cement to be added as per specifications. The Circulation of Cement Bentonite Slurry shall be continued for homogeneous mixing but mixing to be finished before initial setting time of Cement used and bulking time of Cement Bentonite Slurry.

Bleeding of Cement-Bentonite Slurry & Top of Trench: After one day Cement –Bentonite Slurry bleeding occurs which shall cause the top of the slurry to settle down to be top up by additional Cement Bentonite Slurry with required density to be pour to raised up to Cut-Off level.

Removal of stop end pipe: The end tubes shall be taken out gradually with the help of a Crane / Hydraulic Jacks. Adequate care shall be taken during removal of stop end pipe so that no damage is caused to the Slurry placed against them.

Equipment details for one unit:

Sl.no.

Description

Type

Nos.

1

Grabbing Crane

Suitable Capacity

01

2

Bentonite Set up with Pipe Lines

Set

01

3

Grab Suit to Width of Diaphragmwall

Mech./Kelly/Hyd.

01

4

Venture

Set

01

5

Stop End Pipes Suit to width of Diaphragmwall

Set

03

6

Vacseal pump

40HP/30HP

01/01

Stage wise construction sequence

Marking of Diaphragm wall alignment

Construction of Guide walls

Finalization of panel length and marking on the Guide walls

Making of Bentonite slurry
Boring by means of Grabbing method

Cleaning of Excavated Trench

Placing of stop end tubes

Forming of Cement-Bentonite Slurry Diaphragm wall

Bleeding of Cement-Bentonite Slurry & Top of Trench

Removal of stop end pipe

Treatment and Protection of Top of Trench

Safety precaution:
Necessary safety precaution is the part of the job. All workers related with the construction shall be provided with PPE. Periodical checking of cranes, wire shall be done. After boring, the trench shall be covered with iron mesh till other activity starts. In the case of collapse of excavated trench, the same shall be back filled immediately with lean concrete.

Quality Assurance Plan:
As per requirement.

Storage of construction materials:

Cement:
Bagged cement shall be stored in a suitable weather tight warehouse in such a manneras to provide easy access for workers and inspector. Each consignment shall be stacked separately with a flag showing the date of receipt and use of cement bags shall be first come first go basis. In any case the no. of bags in each stack shall not be more than12.Proper record shall be maintained for procurement and consumption of cement bags.

Bentonite:
Bentonite shall be storedin dry and coveredplace near by bentonite slurry tank.